(a) Technical Field
The present invention relates to a catalyst for converting nitrogen oxide into ammonia, and a method for manufacturing the same. More specifically, the present invention relates to a catalyst for converting nitrogen oxide into ammonia, which can efficiently produce NH3 even at a fuel lean region (A/F>14.7, λ (lambda)>1), and a method for manufacturing the same.
(b) Background Art
Recently, with the rapidly increasing oil prices, there is a growing interest in developing a lean burn engine for improving energy efficiency and reducing CO2 emissions. In general, the exhaust gas of a lean burn engine contains excess oxygen, and carbon, thus, carbon monoxide (CO) and hydrocarbon (HC), in the exhaust gas can be easily removed by a conventional after-treatment method. However, nitrogen oxide (NOx) cannot be easily removed from the exhaust gas. Therefore, a Lean NOx Trap (LNT) technology and Selective Catalytic Reduction (SCR) technology using urea (NH3) were developed as after-treatment technologies to reduce NOx produced from the lean burn engine.
The LNT technology is a technology to remove NOx by trapping using a NOx trap material-containing NOx trap catalyst (Lean NOx Trap (LNT) or NOx Storage Catalyst (NSC)).
The SCR technology is a method for spraying an aqueous solution, particularly a urea solution, into an exhaust pipe. The sprayed urea solution is pyrolyzed by heat in the exhaust pipe or catalytically cracked by contact with a catalyst, resulting in the conversion of one molecule of urea into two molecules of NH3. Using this technology, NOx is exhausted as harmless nitrogen and water by reacting NOx in the exhaust gas with the SCR catalyst using the converted NH3.
While both technologies are effective in removing NOx in exhaust gas containing excess oxygen, their application to an actual vehicle is problematic. Namely, the SCR technology using urea needs an infrastructure which can supply urea periodically, to provide NH3 as a reducing agent, and further requires extra equipment for storing and inserting urea into the vehicle.
The LNT technology is also limited because it should use an excess amount of a precious metal to activate the catalyst, it requires a complicated engine operation, and it presents difficulties in removing NOx at higher temperatures (400° C. or more) and lower temperatures (250° C. or less).
Thus, a three-way catalyst (TWC) converter has been broadly used which does not highly modify the existing after-treatment system for a gasoline vehicle. Further, studies have been focused on developing a passive SCR technology, which produces NH3 using the TWC converter and removes NOx using the produced NH3.
However, to carry out the existing passive SCR technology, the air-fuel (A/F) ratio of exhaust gas is decreased to a fuel rich region (A/F<14.7, λ (lambda)<1), which is lower than the theoretical A/F ratio necessary in order to efficiently produce NH3 As such, this existing technology causes further fuel consumption.
Therefore, there is a need for the development of a technology which can efficiently produce NH3 even at a fuel lean region (A/F>14.7, λ>1).